Often times you hear people say that they were shocked and surprised that their spouse was unhappy or wanted a divorce. These same people usually said that their marriage seemed happy, not dissimilar from any of their friends. Are they really surprised or did they simply fail to see the warning signs or deliberately avoid to acknowledge them.
Being comfortable with your spouse is normal but are you ignoring each other. Are you too comfortable in the relationship to care anymore. Are you no longer interested in the other's life. Do you no longer ask how their day was and be honestly interested in the answer. If so, the signs are there but no one bothered to see it.
Taking a spouse for granted and failing to treat them as a person also raises a red flag. Are they just the person who earns the money to pay the bills? Are they just the person who handles the children's needs? Are they just the person who takes care of the house? If so, you have forgotten that your spouse is a person and you should not be surprised if that person mentions divorce because no one wants to be forgotten.
Most times people know that their marriage has issues but fool themselves into thinking that it is either not so bad, or it will get better; that perhaps it is a phase. Using a car analogy, when the "check engine" light comes on what should you do. You can ignore it, but the car will eventually break down. After the car breaks down, can you honestly say that you were surprised.